Start ES embedded server before creating a client.

This resolves a startup-delay problem where Node/Transport protocols
would attempt to find a master before continuing, having no master, would wait until a timeout before continuing (and then finally starting the embedded server!)

Manual test:

    % time bin/logstash -e 'input { generator {  count => 1 } } output { elasticsearch { embedded => true } }'

Before this patch:

     WARN: org.elasticsearch.discovery: [logstash-ds4172-27071-2010]
     waited for 30s and no initial state was set by the discovery

     qbin/logstash -e   20.50s user 0.70s system 45% cpu 46.793 total

After this patch:

    bin/logstash -e   21.53s user 0.52s system 115% cpu 19.070 total